External morphology measurements of several species of the plethodontid genus Aneides

While sexual dimorphism has long received special attention from biologists, derived monomorphism, the condition in which both males and females express similar derived features, has been less well studied.  Historically, the appearance of “male-like” features in females has been explained by the genetic correlation between the sexes.  Recent work emphasizes the importance of studying the independent selective forces on both females and males to understand sexual dimorphism.  Sexual dimorphism and derived monomorphism in the genus Aneides are examined in light of predictions of social selection.  Aneides hardii shows the greatest degree of sexual dimorphism in snout-vent length and head width, with the other species of Aneides less sexually dimorphic. This reduced dimorphism, however, is not a return to an ancestral monomorphic state, but rather exemplifies derived monomorphism because females express traits that were limited in expression to males of ancestral species.  Instead of calling these “male-typical” traits in females, I suggest the term “derived monomorphic” traits, as these traits are typical in these females and “derived monomorphic” can apply to both sexes. Increased attention to studying the patterns and ecological significance of derived monomorphism will shed light on the underlying selective forces, including sexual selection, on both females and males. 

Methods
Preserved animals were measured with calipers and some animals were weighed.

长期以来，性别二态性（sexual dimorphism）一直受到生物学家的广泛关注，但衍生单态性（derived monomorphism）——即雌雄两性均表现出相似衍生性状的状态——的相关研究却相对匮乏。
历史上，学界常以两性间的遗传相关性解释雌性出现“类雄性”特征的现象。近期研究则强调，若要深入理解性别二态性，需同时探究作用于雌性与雄性的独立选择压力。本文基于社会选择（social selection）理论的预测，对无肺螈属（Aneides）内的性别二态性与衍生单态性展开了系统研究。
哈氏无肺螈（Aneides hardii）在吻肛长与头部宽度上表现出最为显著的性别二态性，其余无肺螈属物种的性别二态性相对较弱。不过，这种二态性的减弱并非回归祖先单态状态，而是典型的衍生单态性表现：雌性所表达的性状，在祖先物种中仅局限于雄性个体所拥有。
相较于将此类雌性的性状称为“雄性典型性状”，笔者建议使用“衍生单态性状”这一术语——因为这些性状在该类群雌性中已属常见，且“衍生单态性”可同时适用于雌雄两性。
加强对衍生单态性的分布模式及其生态意义的研究，将有助于阐明作用于雌性与雄性的各类选择压力，其中包括性选择。

方法
本研究使用游标卡尺对保存标本进行形态测量，并对部分个体开展体重称量。